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Is it possible to change the read-only / read-write status of a docker mount at runtime?

I have a docked application that uses the file system to store a lot of state. The application code is contained in the docker image

I am considering an upgrade strategy that involves splitting the volume between two containers, but make sure that no more than one container can write to this filesystem.

The working process:

  • launch container A with / set data rw
  • run container B with / installed data and a newer version of the app
  • stop submitting requests to container A
  • for container A, make / data mount read-only
  • for container B, make / read mount read-write
  • start sending requests to container B
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You can re-mount your volume from inside the container in mode rw

, for example:

mount -o remount,rw /mnt/data

The catch is that the mount syscall is not allowed inside Docker containers by default, so you'll have to run it in privileged mode:

docker run --privileged ...

or enable SYS_ADMIN capability



SYS_ADMIN Performs a number of system administration operations.

docker run --cap-add=SYS_ADMIN --security-opt apparmor:unconfined

(note that I had to add add -security-opt apparmor: unconfined to get this working on Ubuntu).

In addition, reinstalling a volume rw

on a volume ro

can be tricky, as some processes may already have some files inside it open for writing, in which case remount

an error is busy

message will fail .

But my guess is that you can just restart the container instead (since this will be the one that launches the old version of the application).

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